1. Field of the Invention
The invention relates to synchronous motors having a two-phase construction, and in particular to hybrid stepper motors.
Stepping motors can, in general, be considered as a type of synchronous motor in which the windings and magnetic structure have been proportioned such that continuous excitation of one winding is possible while not rotating, without overheating. Further, these motors are generally designed to operate in either direction, so that from any one specific rotor position, resulting from a selected excitation, stepping to an adjacent position in a selected direction is possible by appropriately selecting a change in the excitation of the coils. In order to provide high torque with a minimum of electrical power input, the motors known as hybrid stepper motors incorporate a permanent magnet to provide a field of fixed flux.
In synchronous motors designed for continuous rotation, an important condition for achieving a quality motor is often that the developed torque be quite constant as the shaft rotates, assuming a constant load torque. This is achieved by using a carefully balanced and arranged coil and pole piece structure. Because of the rotational symmetry for both (in a two-phase motor) or all three (in a three-phase motor) structures there is seldom any substantial difference between the torques when one or another of the coils is receiving maximum instantaneous excitation.
A similar requirement arises in the design of hybrid stepping motors, because a difference in the developed torque, depending upon which coil is energized, or in which direction (polarity) it is energized, can lead to a difference in the size of the angle by which the motor moves in consecutive steps. The simplest motor of this type, considered conceptually, would contain two fully independent electromagnetic systems sharing one shaft and a common stator housing. This structure is somewhat analogous to that shown in U.S. Pat. No. 3,855,486 which, however, does not use a permanent magnet. In the most common form of operation, at all times a selected one of the two systems would be energized, and the polarity of energizing the coil would determine which of two angles the rotor moves to. For example, energizing system A may produce stable rotor positioning at angles defined as 0.degree. and 180.degree., while energizing system B produces rotor angles of 90.degree. or 270.degree.. Usually there are large numbers of rotor and stator teeth, so that 360.degree., as referred to above, refers to a simple fraction of one full revolution of the shaft, such as 1/50 revolution.
2. Description of the Prior Art
A first major structural simplification over the basic, two-independent-system motor described above is disclosed in U.S. Pat. No. 4,206,374. In this motor two independent magnetic systems are provided for controlled-excitation flux, but one fixed field flux source is shared by the systems.
Each system includes two rotor discs having a regular tooth pattern about their periphery; for a 1.8.degree. stepping angle, each disc has 50 teeth. Coaxial about each of the rotor discs is a respective stator disc, having the same number of internal teeth. A simple annular stator coil is wound coaxially about the rotor axis, between the stator discs. Surrounding the coil, and in contact with the outer periphery of each stator disc of a system, is a tubular stator portion. Each of the four discs, the tubular portion, and a rotor piece between the rotor discs is made of a soft ferromagnetic material. Thus an exciting current flowing through one of the stator coils will produce flux flowing axially in a first direction in the tubular portion, radially inward through a stator disc and its teeth, across the air gap to the rotor teeth and radially inward, axially through the rotor piece in a direction opposite to the first direction, and radially outward through the other rotor disc, its teeth, the other air gap, the other stator disc teeth and the rest of the disc.
To provide two stable shaft positions, the rotor discs have their teeth in line, while the two stator discs of one stator section have a 1/2 tooth pitch angular shift. Thus the angular alignment of each of the longitudinally aligned rotor-stator disc pairs of one system differs by half a tooth pitch.
The two systems are identical, except that the alignment of the stator sections is shifted by 1/4 tooth pitch, so that at a given shaft position, the teeth of one system are aligned tooth-to-tooth and tooth-to-space, while the other system has its rotor teeth angularly shifted 1/4 of a tooth pitch from the stator teeth.
The two systems share a common fixed flux source: an axially magnetized annular stator magnet fitted between the two stator sections. A third soft magnetic rotor piece is located coaxially with the permanent magnet. Ideally, the fixed flux value, and that produced by the individual stator coil with rated current, are such that when one coil is energized, the fixed and coil fluxes almost cancel each other in the rotor-stator disc pair which are aligned tooth-to-space, and the combined flux in the pair which are tooth-to-tooth is just sufficient to saturate the teeth.
Thus with one coil energized, flux flows through the magnet, splits approximately equally between the two stator discs of the system whose teeth are shifted 1/4 pitch each way from each other, flows through the third rotor piece, and then flows predominantly through the rotor-stator disc pair which is momentarily aligned tooth-to-tooth.
The structure just described is relatively easy to identify and analyze. At first glance, one would expect to see identical stepping distances between all four of the principal positions, corresponding to full excitation of the stator coils, alternately one at a time, and alternating polarity. However, upon careful measurement, a pattern of irregularity in step distances has been identified, but the reasons for this were not recognized.
An improvement to the structure described in the '374 patent is demonstrated in a motor marketed by N. V. Philips' Gloeilampenfabrieken, and further described in Philips Data Handbook Components and Materials. Alignment of the various discs is more quickly and accurately achieved during manufacturing. The rotor of that motor differs in that the two discs of one rotor section are mutually shifted 1/2 tooth pitch, while the two discs of one stator section are in line.
Improvement of the operating characteristics of a stepper motor, when individual stator systems are isolated from each other, is disclosed in the U.S. Pat. No. 3,855,486 referred to above. In this motor, however, it is desired that there be no flux flow between one stator system and another stator system. The desire is that all of the flux produced by one coil shall flow through each of the rotor-stator air gaps associated with that magnetic system, so that maximum torque is obtained. Clearly, also, in the three-phase motor disclosed in that patent, stray flux coupling to adjacent sections would provide a noticeable imbalance from the performance when the center section was energized in comparison with performance when either of the end sections was being energized instantaneously. This motor is thus quite different from the hybrid motor in which flux must flow from one stator section to the other.
An especially short, compact hybrid stepper motor is described in Netherlands Patent application No. 8402543, filed Aug. 20, 1984, to which U.S. patent application Ser. No. 672,021 filed Nov. 16, 1984 (now abandoned) corresponds. For maximum simplicity and compactness of construction, this motor utilizes a single axially magnetized permanent magnet in the rotor, for both systems. The rotor has two identical toothed discs, one to each side of the magnet, with the teeth of one disc aligned 1/4 of a tooth pitch different from the other.
Each stator section is formed from two identical discs, each having two inwardly projecting sectors having teeth facing toward the rotor axis, the sectors each extending less than 90.degree. around the rotor, and the sectors which are part of one stator disc being between the sectors of the other stator disc. A stator coil is wound around the rotor axis, and is disposed between the two discs forming one stator section. Axially extending cylindrical portions around the circumference of each disc function as the "back iron" for the flux resulting from excitation of that coil. The two stator sections are axially adjoining, so that each section has one outer disc, and one inner or middle disc. Thus the flux due to excitation of one section's coil may be thought of as extending radially outward through the first sectors of one disc; along the back iron; inward along the sectors of the other disc; across the air gap to the teeth of the rotor; angularly around the rotor to a location opposite the other (first) sector, and across the air gap to the first sectors. All the stator sectors which are axially in one line have their teeth aligned; but the teeth of one stator sector are disposed 1/2 tooth pitch, around the circumference, from the stator sectors to each side.
The field flux from the permanent magnet flows from one end face of the permanent magnet to the adjoining rotor disc, and then radially outward to the teeth of the stator section discs. Flux flowing to the sectors of the outer disc follows a path radially to the cylindrical portion, then axially along the circumferential back iron to the other stator section, and continues axially to the other outer disc, and radially inward to its air gap, across the gap to the other rotor section, and then axially back to the magnet.
Flux flowing to the inner teeth on the middle disc tends to flow axially to the middle disc of the other stator section, and then radially to the other rotor section.
While this structure seems quite symmetrical, it has been observed that there is a sufficient difference between the torque for one direction of first section coil current, and that for the other direction of coil current, so that for some applications the angular steps are insufficiently identical.